Ion exchange polymer electrolytes and their dispersion in liquid medium are an essential part of fuel cells and other electrochemical applications. In fuel cells, electrochemical reactions occur either in acidic or alkaline media. In acidic environments, proton exchange membranes offer the required combination of adequate longevity and good conductivity at relatively low temperatures (25-100° C.). Whereas fuel cells and electrolytes employ proton exchange membranes, alkaline fuel cells require anion-conducting polymer electrolytes. In alkaline environments, the efficiency of the oxygen reduction reaction is much higher than in acidic conditions, which allows the use of low-cost, abundant electro-catalysts as opposed to precious metal catalysts.
Traditionally, alkaline fuel cells use an aqueous solution of potassium hydroxide as the electrolyte, with typical concentrations of about 30%. A major operating constraint is the requirement for low carbon dioxide concentrations in the oxidant feed stream, as carbon dioxide can result in the formation of carbonate precipitates. One approach for addressing this issue is the use of solid anion conducting membranes. Alkaline fuel cell systems based on such membranes utilize the desirable properties of the solid electrolytes, such as the lack of requirement of liquid electrolyte circulation, less corrosion, and the capability of applying differential pressure and system design simplification.
A significant challenge in the area of alkaline fuel cells is the current lack of anion exchange polymer electrolytes that have i) good electrolyte stability in alkaline media, ii) high anionic conductivity, and iii) good processibility.
Without wishing to be limited by theory, the low stability of anion exchange polymer electrolytes is due to fast hydrolysis of polymer electrolytes in highly basic conditions. The degradation process can be accelerated by electron-withdrawing molecules in the vicinity of cation functional group. Lower ionic conductivity of anion exchange polymer electrolytes, as compared to cation exchange polymer electrolytes, is due to the diffusion coefficient of the hydroxide ion, which is lower than that of protons, and the larger size of cation group in the anion exchange polymer electrolytes, which dilutes the concentration of exchange site. The low processibility of alkyl ammonium cation-based anion exchange polymer electrolytes is due to their low solubility. Alkyl ammonium cation-based (and other cation-based) anion exchange polymer electrolytes may be synthesized by chloride substitution of a —CH2Cl moiety of the polymers. Because the cation form of the polymer electrolytes is directly synthesized via chloride substitution, the resultant cation functionalized polymer electrolytes has limited solubility. The limited solubility has been a significant inhibitor of successful application of alkaline fuel cells.
The prior art teaches that stability of anion exchange polymer electrolytes can be improved by introducing highly basic and bulky cations such as sulfonium, phosphazenium, phosphazene and guanidinium. However, the cation functionality is directly attached to the hydrocarbon-based polymer backbone, which is technically challenging to synthesize. In addition, the stability of fluorinated polymer electrolytes comprising the directly-attached highly basic cations is questionable since electron withdrawing characteristics of fluorine tend to weaken the stability of the bulk cations. However, fluorination of polymer electrolytes is desirable, as it is understood to contribute to high gas permeability.
There exists a need, therefore, for anion conducting polymer electrolytes that are more stable to chemical degradation at high pH than currently available anion exchange polymer electrolytes, that have improved anionic conductivity, and that have better solubility in a dispersing medium, which in turn improves processibility. Additionally, a need exists for methods of fabrication of high performance solid anion exchange membrane fuel cells which comprise the aforementioned anion conducting polymer electrolytes.